This invention relates to electrophotographic reproduction of original documents and more particularly, to an illuminator which provides a uniform radiation output level and which is especially adapted to erase unwanted charge from a photoconductor following document exposure.
In a typical electrophotographic reproduction apparatus an original document is placed on a transparent supporting platen and scanned by an illumination source, causing an image of the document to be projected upon a previously charged, photoconductive surface. A latent electrostatic image is formed on the photoconductor by the selective discharge of areas corresponding to the projected image light pattern. The latent image is then developed, fused and transferred to an output sheet using conventional xerographic techniques. It should be understood that each latent image is exposed and developed within a "frame" corresponding to the dimensions of the original document, or to reduced or enlarged dimensions in a magnification mode of operation. The areas of the photoconductor abutting this framed area are those areas between successive frames (interdocument) and edge areas bordering the frame. These interframe and edge border areas are not exposed to light reflected from the original document and hence retain the charge originally placed on the photoconductor. This charge, if permitted to remain, would be developed in a subsequent development step and may adversely affect the quality of the output copy. In addition, additional toner used in this unwanted development necessitates more frequent toner replenishment.
Various prior art techniques are known to remove the unwanted charge area. These include electrical grounding and reverse charging of the photoconductor as well as redirection of a portion of the illumination from the document illumination source towards the desired areas on the photoconductor (illumination erase). Another known technique is to dedicate an erase lamp for the specific function of illuminating and thus discharging the edge and interframe areas of the photoconductor, such as disclosed in U.S. Pat. No. 3,912,387 and Xerox Disclosure Journal Vol. 7, No. 2, March/April 1982, pp. 107-108. These prior art devices are relatively costly solutions to the erase problem in terms of power requirements and material costs. The output uniformity of the lamp may also prove inadequate for various applications.
It is therefore desirable to provide a low power, low cost illumination charge erase device with improved uniformity characteristics to provide an erase function. According to one aspect of the present invention, a novel inexpensive linear illuminator is so configured as to reduce the illumination source power requirements and improve illumination uniformity at the photo-conductive surface to be erased. This design configuration includes an illuminator assembly comprising a generally wedge-shaped transparent light pipe member having an illumination source disposed so as to couple light into the light pipe member in a direction proceeding towards the narrowing dimensions of the member. The coupled light proceeds along the length of the illuminator via internal reflection from the illuminator surfaces. As the angle made by the internally reflected light projecting the length of the member drops below some critical angle related to the wedge cycle, the light begins to exit from the surface base of the wedge providing a uniform level of illumination emanating from the entire length of the wedge. This uniform escaping illumination provides an exemplary source of energy for a variety of charge erase purposes as will be seen below. An illuminator constructed according to the above principles can also serve as an illumination source for sequentially illuminating portions of a document to be reproduced. For this purpose, higher illumination levels and therefore greater, lamp outputs are required. Both the erase and document illumination functions are discussed below with greater emphasis given to the description of the erase function. More particularly the invention is directed to a linear illumination assembly comprising:
a linear, unitary transparent, member at least one sector of which has a generally wedge-shaped configuration, and,
an illumination source adapted to couple light into said sector.
In one embodiment, the assembly includes a transparent member having a wedge configuration with an illumination source positioned within an aperture in the non-wedge end.